For example, an optical video disk has recess portions called pits arranged in one main surface of the disk so as to form concentric or spiral tracks or an eddy track so that signals can be recorded on the tracks by changing the length and intervals of the pits.
A signal recorded on such a disk is read in a manner so that the disk is irradiated with laser light and the light reflected from the disk or transmitted through the disk is made incident on a photoelectric conversion means for generating a signal corresponding to the quantity of the incident light. In a disk player for reading a signal recorded on such a disk, it is necessary to provide a tracking servo apparatus for causing a laser light spot acting as an information reading point of a pickup to follow correctly a recording track.
As the tracking servo apparatus, for example, it is known to use an apparatus of a so-called three-beam system in which three beams, one being an information reading main beam, and two being tracking-error detecting subsidiary beams disposed on the opposite sides of the main beam, are prepared and arranged so that a line passing through the centers of the respective beams forms a predetermined offset angle with respective to the tracking direction so as to generate an error signal on the basis of a difference in quantity of light between the two subsidiary beams passed through a recording surface of the disk.
It is empirically known that the level of a tracking error signal generated in such a tracking servo apparatus varies in accordance with the relative positional relation between an information reading point and the radial direction of a disk. It is considered that this is because the curvature of a track varies depending on the position on the disk, that is, the curvature of a track is different between the inner and outer circumferential positions on the disk. Thus, the level of a tracking error signal varies substantially linearly corresponding to the relative position of an information reading point of a pickup in the radial direction of a disk so that the level is small at the outer circumference of the disk while large at the inner circumference of the disk. If this tracking error signal is used to perform tracking control, it is difficult to perform the control correctly depending on the relative position of the information reading point of the pickup in the radial direction of the disk.
To eliminate the above difficulty, there has been proposed an apparatus in which the relative position of a pickup in the radial direction of a disk is detected so that control is made to make the gain of an error signal amplifier larger when the detected information reading point is located at the innermost circumference of the disk than that of the error signal amplifier when the detected information reading point is located at the outermost circumference of the disk, as disclosed, for example, in Published Examined Japanese Utility Model Application No. 62-6580.
Such a conventional apparatus has, however, a disadvantage in that it has sometimes been difficult to perform control correctly depending on the disk because of scattering in characteristics of disks, since the gain of an error signal amplifier is controlled only on the basis of the relative position of a pickup.